How to do a channel mask 1) To make it easier for yourself edit the brightness and contrast so the quick selction tool finds it easier to selction the areas you need. Select the subject and right click then select ‘create layer via copy’.
(1) Click the quick mask mode tool (1) and select the
brush tool (2) use the brush over the area you want to change and as you go over it the subject will have a red colour over it (like picture on the right) and then click the quick mask mode which will (2) bring you back to standard mode.
When standard mode is reselected, it will look like how it did when you applied the quick selection tool and then right click and select invert.